1. Technical Field of the Invention
The present invention relates to motor systems that are used in, for example, hybrid vehicles and electric vehicles. In addition, the invention can also be applied to industrial machines and household electrical appliances.
2. Description of the Related Art
In recent years, motors that employ high-performance rare-earth permanent magnets have been widely used in various machines and appliances. At the same time, to avoid resource risk, research has also been made on motors that employ no or less rare-earth permanent magnets.
For example, Japanese Patent Application Publication No. 2004-357489 discloses a motor which employs a consequent-pole rotor so as to reduce the number of rare-earth permanent magnets used in the motor while maintaining high performance of the motor. The consequent-pole rotor includes a plurality of rare-earth permanent magnets, all of which are magnetized in the same direction, and a plurality of opposite poles that are formed by core portions provided between the permanent magnets.
More specifically, in the consequent-pole rotor, all of the permanent magnets are magnetized in the same direction so as to each form a magnet pole. Each of the core portions provided between the permanent magnets forms a consequent pole (or induced pole) that has an opposite polarity to the magnetic poles. That is, for each pole pair, one of the magnetic poles of the pole pair is formed without using a permanent magnet. Consequently, the number of the permanent magnets used in the motor is reduced by half in comparison with the case of forming each of all the magnetic poles with a permanent magnet.
However, the inventor of the present application has found, through an experimental investigation, a problem with the above motor that employs the consequent-pole rotor. That is, in the motor, the permeance of the rotor greatly changes at the boundaries between the magnet poles and the consequent poles. As a result, vibration of a stator core (or armature core) of the motor may be increased, thereby increasing noise of the motor caused by the vibration of the stator core.
More specifically, vibration of the stator core is increased when the change in the permeance of the rotor is in agreement with variation in the spatial magnetic flux created by the armature reaction of the stator (or armature) of the motor. Therefore, to suppress vibration of the stator core, it is necessary to alleviate the variation in the spatial magnetic flux or the change in the permeance of the rotor.
FIG. 7 illustrates an example where the rotor 101 of a conventional full-pitch distributed winding motor is configured as a consequent-pole rotor. In this motor, there are provided three slots of the stator 102 per magnetic pole of the rotor 101. Further, the pitch of the variation in the spatial magnetic flux created by the armature reaction of the stator 102, i.e., the winding pitch j of each of U-phase, V-phase and W-phase windings of the stator 102 is coincident with the pitch k of the boundaries between the magnet poles 104 and the consequent poles 105. Consequently, vibration of the stator core of the stator 102 is amplified, thereby significantly increasing the noise of the motor caused by the vibration of the stator core.
Therefore, it is desired to effectively suppress vibration of the stator core of a motor when the rotor of the motor is configured as a consequent-pole rotor.